Seed germination, seedling emergence, seed persistence and triflusulfuron-methyl sensitivity in Galinsoga parviflora and G. quadriradiata.

Galinsoga quadriradiota Ruiz and Pavon (hairy galinsoga) and Galinsogo parviflora Cav. (smaliflower galinsoga, gallant soldier) are very troublesome weeds in many vegetable row crops in Europe. In order to optimize further management strategies for Galinsoga control in-depth study of germination biology was performed. Germination experiments were conducted to evaluate the impact of light and alternating temperatures on germination of a large set of Galinsoga populations. Seedling emergence was investigated by burying seeds at different depths in a sand and sandy loam soil. Dormancy of fresh achenes harvested in autumn was evaluated by studying germination response in light at 25/20 degrees C with and without nitrate addition. Seed longevity was investigated in an accelerated ageing experiment by exposing seeds to 45 degrees C and 100% relative humidity. A dose-response pot experiment was conducted in the greenhouse to evaluate the effectiveness of triflusulfuron-methyl, applied at the one leaf pair stage, for controlling Belgian Galinsoga populations. Galinsoga seeds required light for germination; light dependency varied among populations. Seedling emergence decreased drastically with increasing burial depth. Maximum depth of emergence varied between 4 and 10mm depending on soil type and population. In a sandy soil, emergence percentages were higher and seedlings were able to emerge from greater depths than in a sandy loam soil. Freshly produced G. parviflora seeds showed a varying but high degree of primary dormancy and were less persistent than G. quadriradiata seeds which lack primary dormancy. Galinsoga parviflora populations were more sensitive to triflusulfuron-methyl than G. quadriradiata populations. The lack of primary dormancy, high seed persistence and lower herbicide sensitivity may explain the higher distribution and abundance of G. quadriradiata over G. parviflora populations in Belgium. Overall, features such as lack of primary dormancy of freshly harvested G. quadriradiata seeds and light dependency for germination may be used to optimize and develop Galinsoga management strategies.

The origin of metamitron resistant Chenopodium album populations in sugar beet.

Chenopodium album L. is a major weed in spring-planted crops in the temperate regions of the world. Since 2000, farmers have reported an unsatisfactory control of this weed in sugar beet fields in Belgium, France and The Netherlands. Frequently, the surviving C. album plants are resistant to metamitron, a key herbicide in this crop. Metamitron resistance in C. album is caused by a Ser264 to Gly mutation in the psbA gene on the chloroplast genome, which prevents binding of metamitron to its target site. This mutation causes also resistance to other herbicides with a similar mode of action, like metribuzin -applied in potato- and atrazine in particular. Atrazine has been applied very frequently in maize in the 1970s and the 1980s, but is now banned in Europe due to environmental reasons. The persistent use of atrazine in maize confronted Belgian and other European farmers in the early 1980s with atrazine resistant C. album with the same Ser264 to Gly mutation. The problems with atrazine resistant C. album disappeared when other herbicides were applied in maize. Unfortunately, this is not the case for metamitron resistant C. album in sugar beet, because no replacement herbicide is readily available. The history of atrazine use in maize brought up a question concerning the origin of the current metamitron resistant C. album populations. Have these populations been selected locally by regular use of metamitron in sugar beet or did the selection occur earlier by atrazine use when maize was grown in the same fields? This would have serious implications regarding the reversibility of herbicide resistance. Therefore, soil samples were collected on 16 fields with different histories: five fields with an organic management over 25 years, two fields with a history of atrazine resistant C. album, five fields with metamitron resistant C. album in sugar beet and four fields which were under permanent grassland for 10 years, preceded by a regular rotation in which sugar beet was a key crop. The seeds of C. album were extracted from the soil and germinated on a germination table. Germinated seeds were allowed to grow in a growth chamber. Metamitron resistance was determined by a chlorophyll fluorescence test and leaf material was sampled for AFLP-analysis. For all fields, estimations were made of the size of the seed bank (i.e. an indirect estimate of population size), the frequency of resistant plants and the genetic diversity of resistant and susceptible populations. The results indicate that herbicide-resistant C. album populations are persistent and maintain their adaptive capacity, challenging future management of metamitron resistant C. album.

Sensitivity of locally naturalized Panicum species to HPPD- and ALS-inhibiting herbicides in maize.

Until recently the Panicum species Panicum schinzii Hack. (Transvaal millet), Panicum dichotomiflorum Michx. (Fall panicum) and Panicum capillare L. (Witchgrass) were completely overlooked in Belgium. Since 1970, these species have gradually spread and are now locally naturalized and abundant in and along maize fields. One of the possible raisons for their expansion in maize fields might be a lower sensitivity to postemergence herbicides acting against panicoid grasses, in particular those inhibiting 4-hydroxyphenyl pyruvate dioxygenase (HPPD) and acetolactate synthase (ALS). A dose-response pot experiment was conducted in the greenhouse to evaluate the effectiveness of five HPPD-inhibiting herbicides (sulcotrione, mesotrione, isoxaflutole, topramezone, tembotrione) and two ALS-inhibiting herbicides (nicosulfuron, foramsulfuron) for controlling Belgian populations of P. schinzii, P. dichotomiflorum and P. capillare. Shortly after sowing, half of all pots were covered with a film of activated charcoal to evaluate foliar activity of the applied herbicides. In another dose-response pot experiment, sensitivity of five local P. dichotomiflorum populations to HPPD-inhibitors and nicosulfuron was investigated. Finally, the influence of leaf stage at time of herbicide application on efficacy of topramezone and nicosulfuron for Panicum control was evaluated. Large interspecific differences in sensitivity to HPPD-inhibiting herbicides were observed. Panicum schinzii was sensitive (i.e., required a dose lower than the maximum authorized field dose to achieve 90% reduction in biomass) to tembotrione but moderately sensitive (i.e. required maximum field dose) to topramezone and poorly sensitive (i.e. required three-fold higher dose than maximum field dose) to mesotrione and sulcotrione. However, P. dichotomiflorum, a species that morphologically closely resembles P. schinzii, was sensitive to mesotrione and topramezone but moderately sensitive to tembotrione. All Panicum species were sensitive to low doses of nicosulfuron and foramsulfuron. The relative contribution from soil activity to weed control resulting from postemergence applications was important for isoxaflutole, sulcotrione, tembotrione and mesotrione but not for topramezone, nicosulfuron and foramsulfuron. Naturalized Panicum dichotomiflorum populations exhibited differential herbicide sensitivity profiles. Panicum schinzii, P. capillare and P.dichotomiflorum showed a progressive decrease in sensitivity to topramezone and nicosulfuron during seedling development. A satisfactory postemergence control of Panicum species in the field will require appropriate choice of herbicide and dose, as well as a more timely application (i.e. before weeds reach the four leaves stage).

Local spread of metamitron resistant Chenopodium album L. patches.

Molecular markers can provide valuable information on the spread of resistant weed biotypes. In particular, tracing local spread of resistant weed patches will give details on the importance of seed migration with machinery, manure, wind or birds. This study investigated the local spread of metamitron resistant Chenopodium album L. patches in the southwest region of the province West-Flanders (Belgium). During the summer of 2009, leaf and seed samples were harvested in 27 patches, distributed over 10 sugar beet fields and 1 maize field. The fields were grouped in four local clusters. Each cluster corresponded with the farmer who cultivated these fields. A cleaved amplified polymorphic sequence (CAPS) procedure identified the Ser264 to Gly mutation in the D1 protein, endowing resistance to metamitron, a key herbicide applied in sugar beet. The majority of the sampled plants within a patch (97% on average) carried this mutation. Amplified fragment length polymorphism (AFLP) analysis was performed with 4 primer pairs and yielded 270 molecular markers, polymorphic for the whole dataset (303 samples). Analysis of molecular variance revealed that a significant part of the genetic variability was attributed to variation among the four farmer locations (12 %) and variation among Chenopodium album patches within the farmer locations (14%). In addition, Mantel tests revealed a positive correlation between genetic distances (linearised phipt between pairs of patches) and geographic distances (Mantel-coefficient significant at p = 0.002), suggesting isolation-by-distance. In one field, a decreased genetic diversity and strong genetic relationships between all the patches in this field supported the hypothesis of a recent introduction of resistant biotypes. Furthermore, genetic similarity between patches from different fields from the same farmer and from different farmers indicated that seed transport between neighbouring fields is likely to have an important impact on the spread of metamitron resistant biotypes.

Sensitivity of Echinochloa muricata and Echinochloa crus-galli to HPPD- and ALS-inhibiting herbicides in corn.

Until recently Echinochloa muricata var. microstachya Wiegand (rough barnyardgrass), an alien species native to North America, was completely overlooked in Belgium due to its close morphological resemblance to Echinochloa crus-galli (L.) P. Beauv. (barnyardgrass). E. muricata var. microstachya has gradually spread and is now locally naturalized and abundant in and along maize fields. One of the possible reasons for its expansion in maize fields, besides e.g. the lack of crop rotation, might be a lower sensitivity to postemergence herbicides acting against panicoid grasses, in particular 4-hydroxyphenyl pyruvate dioxygenase (HPPD)-inhibiting herbicides and acetolactate synthase (ALS) inhibiting herbicides. Dose-response pot experiments were conducted in the greenhouse to evaluate the effectiveness of four HPPD-inhibitor herbicides [topramezone (ARIETTA), mesotrione (CALLISTO), tembotrione (LAUDIS), sulcotrione (MIKADO) and the ALS-inhibitor herbicide nicosulfuron (KELVIN) for controlling local populations of E. crus-galli and E. muricata. Pots were planted with 25 seeds, thinned afterwards to 5 plants (one week after sowing) and irrigated by overhead sprinklers. Herbicides were applied at the 3-4 leaf stage (BBCH stage 13-14). Fresh biomass was harvested 28 d after treatment. In another dose-response pot experiment, the influence of leaf stage at time of herbicide application on efficacy of topramezone for (rough) barnyardgrass control was evaluated. Sensitivity to HPPD-inhibitor herbicides topramezone and sulcotrione was significantly lower for E. muricata populations than for E. crus-galli populations. However, nicosulfuron sensitivity of both species was similar. Compared to E. crus-galli, sensitivity of E. muricata to topramezone was more dependent on leaf stage. Due to the intragenus variability in sensitivity to HPPD-inhibitor herbicides, higher awareness is required for presence of E. muricata plants in maize fields in order to avoid insufficient "barnyardgrass" control.

Distribution of metamitron-resistant Chenopodium album L. in Belgian sugar beet.

Chenopodium album L. (fat-hen) with a Ser264-Gly mutation is resistant to photosystem II-inhibiting herbicides like the triazinone metamitron, a key herbicide in sugar beet. In recent years, this resistant biotype may cause unsatisfactory weed control in Belgian sugar beet. However, the dimension of the problem was yet unknown. Therefore, a survey was conducted in 2008 covering the whole Belgian sugar beet area. In randomly selected fields, C. album plants surviving weed control were counted and sampled. First, the number of surviving plants was used to estimate the prevalence of fields with unsatisfactory control and to classify the surveyed fields. Then, the share of the resistant biotype in each field was determined with cleaved amplified polymorphic sequence-analysis (CAPS-analysis) on sampled leaves. Finally, all results were visualised on the map of Belgium. Twenty percent of the fields had more than 500 surviving plants per hectare and were thus classified as fields with unsatisfactory C. album control. The resistant biotype was present in 95% of these fields and even in 74% of the sampled fields with good weed control. No pattern was found during mapping. These results indicate that the metamitron-resistant biotype has spread over the whole sugar beet area but that it is not (yet) causing severe problems in every field. To get a more accurate estimation of the portion of resistant plants in the field and the effect of herbicide treatment on this biotype, an elaborate survey will be conducted in 2010 on fields that have both untreated and treated plots installed.

Weed seed bank response to 12 years of different fertilization systems.

Fertilizer amendments can impact weed populations in a variety of ways. This study evaluated the effects of 12 year-long applications of different fertilization systems on size and composition of the weed seed bank in a conventionally managed maize monoculture field. Fertilization systems included all factorial combinations of two dairy cattle slurry rates, three vegetable, fruit and garden waste (VFG) compost rates, and three synthetic N fertilizer rates. Soil samples were taken in each subplot in May 2008 after sowing and prior to herbicide application. Residues recovered from soil samples were tested for weed seedling emergence to characterize soil seed banks. Total weed seed bank density was affected by mineral N fertilization but not by compost or animal slurry application. Weed seed bank composition was related to compost amendment and mineral N fertilization. Annual compost amendments reduced seed bank density of some persistent species (e.g., Chenopodium album and Solanum nigrum) irrespective of mineral N fertilization. Compost is a promising tool for incorporation into integrated weed control strategies aimed at reducing weed seed bank persistence.

Chlorophyll fluorescence protocol for quick detection of triazinone resistant Chenopodium album L.

Sugar beet growers in Europe are more often confronted with an unsatisfactory control of Chenopodium album L. (fat-hen), possibly due to the presence of a triazinone resistant biotype. So far, two mutations on the psbA-gene, i.e. Ser264-Gly and Ala251-Val, are known to cause resistance in C. album to the photosystem II-inhibiting triazinones metamitron, a key herbicide in sugar beet, and metribuzin. The Ser264-Gly biotype, cross-resistant to many other photosystem II-inhibitors like the triazines atrazine and terbuthylazine, is most common. The second resistant C. album biotype, recorded in Sweden, is highly resistant to triazinones but only slightly cross-resistant to terbuthylazine. Since farmers should adapt their weed control strategy when a resistant biotype is present, a quick and cheap detection method is needed. Therefore, through trial and error, a protocol for detection with chlorophyll fluorescence measurements was developed and put to the test. First, C. album leaves were incubated in herbicide solution (i.e. 0 microM, 25 microM metribuzin, 200 microM metamitron or 25 microM terbuthylazine) during three hours under natural light. After 30 minutes of dark adaptation, photosynthesis yield was measured with Pocket PEA (Hansatech Instruments). In Leaves from sensitive C. album, herbicide treatment reduces photosynthesis yield due to inhibition of photosynthesis at photosystem II. This results in a difference of photosynthesis yield between the untreated control and herbicide treatment. Based on the relative photosynthesis yield (as a percentage of untreated), a classification rule was formulated: C. album is classified as sensitive when its relative photosynthesis yield is less than 90%, otherwise it is resistant. While metribuzin, and to a lesser extent, metamitron treatment allowed a quick detection of triazinone resistant C. album, terbuthylazine treatment was able to distinguish the Ser264-Gly from the Ala251-Val biotype. As a final test, 265 plants were classified with the protocol. Simultaneously, a CLeaved Amplified Polymorphic Sequence (CAPS)-analysis was conducted on the same plants to verify the presence of the Ser264-Gly mutation. Only one mismatch was found when results of both detection methods were compared. The test results illustrate that this protocol provides a reliable, quick and cheap alternative for DNA-analysis and bio-assays to detect the triazinone resistant C. album biotypes.

Resistance of Chenopodium albumto photosystem II-inhibitors.

Chenopodium album L. (fat-hen), a highly competitive and very prolific species, is a common weed in most spring- and summer-sown crops such as maize, sugar beet and vegetables. In the late seventies, C. album stepped into the limelight as a problem weed in maize. Frequent use of atrazine in maize monoculture did select for plants having a Ser-264-Gly mutation on the psbA gene resulting in atrazine-resistance and cross-resistance to other Photosystem (PS) II-inhibitors. The psbA gene encodes the D1 protein of PS II which is the target site of PS II-inhibitors. Introduction of new herbicides made it possible to control this atrazine-resistant biotype in maize, which allowed C. album to fade into the background again until it resurfaced some years ago as a problem weed in European sugar beet (Belgium, France, The Netherlands and Sweden). Greenhouse bioassays at Ghent University revealed that the unsatisfactory control of C. album in sugar beet is due to resistance to the triazinone metamitron, a key herbicide in sugar beet. The expected cross-resistance to atrazine and metribuzin was found in all populations except for a Swedish one, which is highly resistant to metamitron and metribuzin but not to atrazine. DNA sequence analysis confirmed the presence of a Ser-264-Gly mutation for all populations that are both metamitron- and atrazine-resistant. The Swedish population has an Ala-251-Val mutation on the psbA gene explaining its aberrant (cross)-resistance profile. The occurrence of C. album biotypes with resistance to metamitron but different genotypes and cross-resistance profiles could raise the question which herbicide(s) did select for the resistance. In Sweden, having no history of atrazine use, the triazinones metamitron, used in sugar beet, and metribuzin, used in rotational potato, could have selected for resistance. In Belgium, at least three different herbicides and/or crop rotations could have contributed to resistance development: (1) a record of continuous use of atrazine in maize resulting in triazine-resistant C. album in the seed bank, (2) metamitron use in sugar beet and (3) metribuzin use in potato.

Chlorophyll fluorescence tests for monitoring triazinone resistance in Chenopodium album L.

Recently, fat-hen (Chenopodium album L.) biotypes resistant to metamitron, a key herbicide in sugar beet, were recorded. Pot experiments revealed that these biotypes showed cross-resistance to metribuzin, a triazinone used in potato. Greenhouse and laboratory experiments were performed to develop resistance monitoring tests, so that resistant biotypes can be detected quickly and farmers may adapt their weed management. Resistant and susceptible biotypes were grown in a greenhouse under conditions of natural and artificial light at an intensity of 100 micromol photons m(-2) s(-1). Leaves were collected and, immersed in a solution of 1000 microM metamitron and 500 microM metribuzin, exposed to natural and artificial light (1000, 750 and 100 micromol photons m(-2) s(-1) respectively). After this, chlorophyll fluorescence measurements were carried out. The results revealed that the photosynthetic electron transport of metamitron- and metribuzin-incubated leaves of resistant biotypes decreased less than that of the incubated Leaves of susceptible biotypes. The differences between the metribuzin-incubated leaves of the susceptible and resistant biotypes were larger than those observed with the metamitron-incubated leaves. The aim of the experiments was to optimise the chlorophyll fluorescence test and to find a sufficiently high correlation between the results of the pot experiments and the chlorophyll fluorescence measurements.

Effect of selected sugar beet herbicides on germination of various Chenopodium album populations.

Seeds of various fat-hen populations (Chenopodium album L.), mostly originating from sugar beet fields, were subjected to treatments with the following herbicides: metamitron, acetochlor, dimethenamid-P and S-metolachlor. Herbicides were applied either incorporated into a sandy Loam soil (2005-2007) and/or on filter paper in Petri dishes (2006-2007). Results between experiments were highly contrasting. Soil applications of metamitron, acetochlor and S-metolachlor were stimulating germination in the 2005 experiments, whereas in the 2006-2007 experiments effects were ranging from slightly stimulating to highly inhibitory.

Metamitron-resistant Chenopodium album from sugar beet: cross-resistance profile.

In recent years, in several of the Belgian sugar beet growing regions, farmers have been confronted with unsatisfactory control of fat hen (Chenopodium album L.). Greenhouse bioassays conducted on reference C. album populations and on "suspected" populations from sugar beet fields where poor fat hen control had been observed, revealed that all "suspected" populations were resistant to metamitron, a key herbicide in the modern low rate weed control programs in sugar beet. These metamitron-resistant biotypes were all cross-resistant to atrazine. Since cross-resistance, particularly negative cross-resistance or reversed resistance, is known to play a major role in resistance management, other herbicides used in sugar beet and/or in rotational crops were tested to determine the cross-resistance profile of metamitron-resistant biotypes. Greenhouse bioassays were conducted using herbicides from different chemical families representing different modes of action. Cross-resistance was found for metribuzin, lenacil and chloridazon, all HRAC Group C1 herbicides that inhibit photosynthesis at PS II. The metamitron-resistant C. album populations examined showed negative cross-resistance to S-metolachlor (HRAC Group K3: inhibition of cell division), prosuifocarb (Group N: lipid synthesis, not AC-Case, inhibition), aclonifen and clomazone (both Group F3: inhibition of carotenoid biosynthesis).

Resistance to metamitron in Chenopodium album from sugar beet. a tale of the (un)expected?

Metamitron is a key herbicide in modern low rate weed control programs in sugar beet. Fat hen (Chenopodium album, CHEAL) is a common, highly competitive, weed in sugar beet and one of the targets of metamitron. Recently, unsatisfactory control of fat hen has been reported in several sugar beet fields situated in various regions in Belgium. Weather conditions as well as the mere fact of using low rate systems have been blamed for these poor performances. To address the question "Is the recently recorded poor control of C. album due to decreased sensitivity to metamitron", greenhouse bioassays were carried out. A first experiment was conducted applying metamitron (0, 350, 700 and 1,400 g ai/ha) postemergence to three "suspected" C. album populations originating from sugar beet fields with unsatisfactory control by standard metamitron based treatment schemes ('Ligne', 'Outgaarden' and 'Boutersem I' respectively) and to one sensitive population originating from an untreated garden site ('Gent'). In a second experiment seven population, five "suspected" fat hen populations from sugar beet fields ('Boutersem I', 'Boutersem II', 'Postel', 'Vissenaken' and 'Kortessem' respectively), one sensitive reference population 'Herbiseed' and one atrazine-resistant reference population 'ME.85.01', were submitted to metamitron (0, 1, 2 and 4 mg ai/kg air-dry soil) and atrazine (1.5 mg ai/kg air-dry soil) preplant incorporated. All "suspected" C. album populations displayed a significantly lower sensitivity to metamitron compared to the sensitive populations ('Gent' and 'Herbiseed') that never had been exposed to this herbicide. As target site cross-resistance of atrazine-resistant C. album, selected by atrazine in maize, to metamitron has been known for a long time, cross-resistance of C. album populations in sugar beet grown on fields with "maize - atrazine" containing rotations might be expected to appear. The outcome of the experiment with atrazine preplant incorporated was the confirmation of resistance in all "suspected" populations ('Boutersem I', 'Boutersem II', 'Postel', 'Vissenaken' and 'Kortessem'). However, some "suspected" populations came from fields with no background of cropping with maize and use of atrazine. So, the question remains whether these triazine-resistant C. album had been imported, e.g. with slurry, or the rather unexpected possibility that metamitron itself did select for metamitron-resistant biotypes bearing cross-resistance to atrazine, had become reality.

Soil activity and persistence of sulcotrione and mesotrione.

Greenhouse bioassays were set up using a small pot test method to determine the intrinsic sensitivity of different plant species to sulcotrione and mesotrione applied in a sandy loam soil. Herbicides were applied over an appropriate concentration range. After a 2-3 week test period, foliage fresh weight was determined. Data were subjected to a non-lineair regression analysis. Using the regression equations, ED50-values (herbicide concentrations that cause 50 percent foliage fresh weight reduction) were calculated for each combination of crop species and herbicide. To determine which replacement crops might be grown in case of failure of a crop treated with one of these herbicides, field persistence experiments were conducted over the 1993-2003 period for sulcotrione and the 1998-2003 period for mesotrione at the Experimental Farm, Biocentre Agri-Vet, Ghent University at Melle. Herbicides were applied in spring (about mid-March) on a bare soil; untreated control strips were included. Replacement crops were sown or planted approximately five weeks after herbicide applications. Visual estimations of crop injury were recorded at several intervals from sowing and fresh matter yield of plant parts was determined. Based on these data, crops were ranked according to their degree of sensitivity to either sulcotrione or mesotrione. Maize is very tolerant to both herbicides, although in some years, temporary injury could be seen in the field experiments. Italian rye-grass and fibre flax are tolerant crops; in field experiments a slight, temporary injury could be noticed in some years. Winter wheat displayed a high degree of tolerance to mesotrione (in both experiment types): however this crop was less tolerant to sulcotrione especially in the bioassay experiment. Based on its ED50-value, black salsify is tolerant to sulcotrione but under field conditions, the selectivity of this herbicide is quite variable; tolerance to mesotrione is moderate. Turnip and witloof chicory are clearly sensitive to mesotrione and sulcotrione whereas sugar beet, red clover and lettuce are extremely sensitive to both herbicides in both experiment types. Bioassays and field experiments provide a detailed and complete information about soil activity and persistence of both herbicides.

Field experiences with recent ALS-inhibitors on herbicide resistant blackgrass (Alopecurus myosuroides Huds.).

In the growing season 2002-2003 two field experiments were carried out in winter wheat on the heavy clay soil of the coastal polder area at Zevekote to study the response of blackgrass (Alopecurus myosuroides Huds.) resistant or somewhat less sensitive to a wide variety of herbicides (clodinafop-propargyl, fenoxaprop-P-ethy1; flupyrsulfuron-methyl+metsulfuron-methyl, propoxycarbazone-sodium; isoproturon) representing various modes of action. In Experiment 1, preemergence applications of isoproturon+diflufenican (1500+187.5 g/ha) and isoproturon+diflufenican+flurtamone (1250+100+250 g/ha) respectively were followed in mid-March (Zadoks: 23) by one of the following treatments: none, propoxycarbazone-sodium + vegetable oil (42 g/ha + 1 l/ha), mesosulfuron-methyl + iodosulfuron-methyl-sodium (+mefenpyr-diethyl) + vegetable oil (15+3 (+45) g/ha + 1 l/ha), clodinafop-propargyl (+cloquintocet-mexyl) {60 (+15) g/ha} and flupyrsulfuron-methyl+metsulfuron-methyl (10+5 g/ha). Systems based on clodinafop-propargyl, propoxycarbazone-sodium or flupyrsulfuron-methyl+metsulfuron-methyl resulted in poor supplementary control of blackgrass compared to preemergence herbicide application only. On the contrary, systems based on postemergence application of mesosulfuron-methyl + iodosulfuron-methyl-sodium resulted in excellent control. In most cases the few surviving plants failed to produce inflorescences. In Experiment 2, fall applications in the 3 leaves stage (Zadoks: 13) of prosulfocarb + isoxaben (4000+75 g/ha), flufenacet + diflufenican + isoxaben (240+120+75 g/ha) and flufenacet + pendimethalin + chlorotoluron (180+900+1000 g/ha) respectively were followed in mid-March (Zadoks: 23) by one of the following treatments: none, propoxycarbazone-sodium + vegetable oil (42 g/ha+l l/ha), mesosulfuron-methyl + iodosulfuron-methyl-sodium (+mefenpyr-diethyl) + vegetable oil {15+3 (+45) g/ha + 1 l/ha}, clodinafop-propargyl (+cloquintocet-mexyl) {60 (+15) g/ha}, flupyrsulfuron-methyl + metsulfuron-methyl (10+5 g/ha) and isoproturon+diflufenican (1000+125 g/ha). As in Experiment 1, systems based on clodinafop-propargyl, propoxycarbazone-sodium or flupyrsulfuron-methyl+metsulfuron-methyl resulted in poor additional control of blackgrass compared to herbicide application in the fall only. Comparable poor levels of blackgrass control could be observed with isoproturon+diflufenican. Mesosulfuron-methyl + iodosulfuron-methyl-sodium resulted in excellent control comparable to that recorded in Experiment 1.

Soil metabolism of isoxaflutole in corn.

The herbicide isoxaflutole 1 (5-cyclopropyl-4-isoxazolyl)[2-(methylsulfonyl)-4-(trifluoromethyl)phenyl]-methanone) has been applied preemergence at the rate of 125 g ha(-1) on corn crops grown on fields located in regions different as to their soil textures. Its metabolite diketonitrile 2 (2-cyano-3-cyclopropyl-1-(2-methylsulfonyl-4-trifluoromethylphenyl)propane-1,3-dione)-which is the herbicide's active compound-and its nonherbicide metabolite 3 (2-methylsulfonyl-4-trifluoromethylbenzoic acid) were measured in the 0-10 cm surface soil layer of the corn crops after the treatment and until the harvest. At the opposite of what occurred in plant shoots, the transformation of isoxaflutole 1 into diketonitrile 2 was not immediate in soil. In the 0-10 cm surface soil layer, this transformation occurred progressively according to an apparent second-order kinetics, and the soil half-lives of isoxaflutole 1 self were comprised between 9 and 18 days. The adsorption of isoxaflutole 1 onto the solid phase of the soil and its organic matter should explain the stabilization effect of soil, increased by the application of fresh organic fertilizer. The sum of the concentrations of isoxaflutole 1 and diketonitrile 2 disappeared in the 0-10 cm surface soil layer according to an apparent first-order kinetics, and the soil half-lives of this sum were comprised between 45 and 65 days. The sum of the concentrations of isoxaflutole 1 and of its metabolites diketonitrile 2 and acid 3 did not account for the amount of isoxaflutole 1 applied. The discrepancy increased with the delay after the application, showing that the acid 3 was further metabolized in soil into common nontoxic products, and ultimately into CO2. The conjunction of the adsorption of isoxaflutole and its metabolites (which reduced their mobilities) onto the soil and its organic matter, and their further metabolism should explain why isoxaflutole and its metabolites were not detected in the 10-15 and 15-20 cm surface soil layers during the crops.